1. Technical Field
The present invention relates to an apparatus for detecting a failure of an exhaust gas sensor provided in an exhaust system of an internal combustion engine.
2. Description of Related Art
A catalyst converter for purifying exhaust gas is provided in an exhaust system of an internal combustion engine of a vehicle. When the air-fuel ratio of air-fuel mixture introduced into the engine is lean, the catalyst converter oxidizes HC and CO with excessive oxygen included in the exhaust gas. When the air-fuel ratio is rich, the catalyst converter reduces Nox with HC and CO. When the air-fuel ratio is in the stoichiometric air-fuel ratio region, HC, CO and Nox are simultaneously and effectively purified.
An exhaust gas sensor is provided downstream of the catalyst converter. The exhaust gas sensor detects the concentration of oxygen included in the gas that is discharged into the exhaust system. The detection value of the exhaust gas sensor is used for various control of the internal combustion engine. If response of the exhaust gas sensor deteriorates, the internal combustion engine may not be appropriately controlled, which may cause deterioration in the operating state of the engine.
Japanese Patent No. 2812252 discloses a method for detecting an abnormal state of an exhaust gas sensor based on the differential of the output of the exhaust gas sensor. When the number of times that the differential of an output of the exhaust gas sensor is great is less than a predetermined value, it is determined that the exhaust gas sensor is in an abnormal state. The method is based on the knowledge that the time-differential of the sensor output is often far from zero when the exhaust gas sensor is normal, while the time-differential of the sensor output is always close to zero when the exhaust gas sensor is faulty.
Since the catalyst converter and the exhaust gas sensor are disposed in series, the time-differential of the output of the exhaust gas sensor is influenced by delay characteristics (low pass characteristics) of the catalyst. That is, the output of the exhaust gas sensor varies according to the degree of deterioration of the catalyst. In the above conventional scheme of analyzing variations in the time-differential of the output of the exhaust gas sensor, it is difficult to determine whether such variations are caused by changes in response characteristics of the catalyst or by changes in response characteristics of the exhaust gas sensor. Therefore, there is a need for an apparatus and a method capable of detecting a failure of an exhaust gas sensor as distinguished from deterioration of catalyst.